The present invention relates to a process for the impregnation of an iron product with a surface comprising a hard wear-resistant material.
A wide variety of techniques are known for the impregnation of iron with a hard wear-resistant surface. Such techniques include flame spray coating and plasma spray coating. However, each of these spray coating techniques suffer from problems associated with the spalling of surface layers during the coating process and during service as well as the particularly large expense associated with the use of these techniques.
Cast-In-Carbides are also known in which carbide particulates are placed within a mold and molten iron is then cast. See, for example, the discussion within U.S. Pat. No. 4,119,459 to Ekemar et al. It is difficult, however, with such castings to accurately maintain the carbide particles in the desired location and in a regular distribution pattern.
In addition, certain cast-on hard surfacing techniques for use with polystyrene patterns are also known in the art. See, for example, the discussion in Hansen et al, "Application of Cast-On Ferrochrome-Based Hard Surfacings to Polystyrene Pattern Castings," Bureau of Mines Report of Investigations 8942, U.S. Department of the Interior, 1985.
With the process discussed in Hansen et al, a paste comprising a binder and the desired hard material, such as tungsten carbide powder, is applied to those surfaces of a polystyrene pattern which correspond to wear-prone surfaces of the resulting casting. A refractory coating is then applied on the entire pattern prior to casting the metal, the process being known as "evaporative pattern casting" process or EPC process.
However, this process suffers from problems associated with the low reliability of the bond formed between the wear resistant layer, e.g., tungsten carbide, and the foam pattern which is predominantly caused by the failure of the nearly dry paste to wet the foam surface sufficiently. Because of this failure, sometimes the iron does not penetrate the layer before the iron solidifies and, thus, instead of impregnating the iron, the carbide spalls off the product.
This process is also complex and inefficient and thus cannot be effectively employed for large scale production.
In addition, the prior art methods are particularly unsuitable for the production of a wear resistant layer of larger thicknesses. In particular, if larger thicknesses are employed in prior art processes, liquid metal penetration becomes difficult to achieve. Thus, the carbide particles are not trapped by the metal and they tend to spall off.
It is known iron castings made by EPC process have lower mechanical properties than sand (or core) cast products due to presence of carbon defects. Also the EPC process requires special care to minimize distortions in castings.
Accordingly, the need still exists for a method of impregnating iron surfaces with a hard wear-resistant material, particularly when employing larger size particles, to produce larger thicknesses.